Since its birth in 1940s, catalytic cracking has been the main process for lightening heavy oils. One of the reasons is that the raw materials thereof come from a variety of sources, including gas oil, atmospheric residue, deasphalted oil of vacuum residue, or partially doped vacuum residue. Secondly, the production solutions are flexible and may be the fuel-type, or fuel-chemical engineering type, e.g. producing more gasoline, diesel oil, propylene and the like. Thirdly, the product properties can be correspondingly adjusted by modifying the catalyst formulation and changing the processing parameters, e.g. increasing the octane number of gasoline, reducing the olefin content in gasoline and the like.
The conventional catalytic cracking techniques are primarily used for producing gasoline, wherein the gasoline yield is as high as more than 50 wt. %. At the beginning of 1980s, unleaded gasoline drives the catalytic cracking techniques to the direction of producing gasoline with high octane number. Thus the technological conditions of catalytic cracking and catalyst type greatly change. In the technological aspect, there primarily include increasing the reaction temperature, reducing the reaction time, increasing the reaction severity level, inhibiting the hydrogen transfer and overcracking reactions and improving the efficiency of contacting the oil and gas at the bottom of the riser with the catalyst. In the catalyst aspect, the catalysts formed by combining USY-type zeolites with inert substrate or active substrate and compounded from different types of zeolites are developed.
The catalytic cracking techniques have achieved the developments above, satisfied the requirements on Pb-free gasoline and increase the octane number of gasoline. However, either the change of the technological conditions or use of novel zeolite catalyst to increase the octane number of gasoline increases the octane number of gasoline by increasing the olefin content in the gasoline components. The olefin content in the current gasoline components ranges from 35 to 65 wt. %, which is greatly different from the requirements on the olefin content in the new gasoline. The olefin content in the liquefied petroleum gas composition is higher and about 79 wt. %, wherein butene is in a content of several times of the content of isobutane and cannot be used as the alkylation raw materials.
ZL99105904.2 discloses a catalytic conversion method for preparing isobutane and isoalkane-enriched gasoline comprising feeding the preheated feedstock oil into a reactor having two reaction zones, contacting with a hot cracking catalyst, wherein the reaction in the first reaction zone is conducted at 530-620° C. for 0.5-2.0 s; the reaction in the second reaction zone at 460-530° C. for 2-30 seconds, separating the reaction product, stripping the spent catalyst to be regenerated, feeding the stripped catalyst into the regenerator, coking and recycling. The content of isobutane in liquefied petroleum gas produced according to the method in the invention is 20-40 wt. %; the content of isoalkane in the gasoline composition is 30-45 wt. %; and its olefin content is deceased to less than 30 wt. %; the RON ranges from 90 to 93; the MON ranges from 80 to 84.
ZL99105905.0 discloses a catalytic conversion process for producing propylene, isobutane and isoalkane-enriched gasoline, comprising feeding the preheated feedstock oil into a reactor having two reaction zones, contacting with a hot cracking catalyst, wherein the reaction in the first reaction zone is conducted at 550-650° C. for 0.5-2.5 s; the reaction in the second reaction zone at 480-550° C. for 2-30 seconds, separating the reaction product, stripping the spent catalyst to be regenerated, feeding the stripped catalyst into the regenerator, coking and recycling. The yield of the liquefied petroleum gas can reach 25-40 wt. %, in which the contents of propylene and isobutane are about 30 wt. % and 20-40 wt. % respectively. The yield of gasoline can reach 35-50 wt. %, in which the isoalkane is 30-45 wt. %
ZL99105903.4 discloses a riser reactor for fluidized catalytic conversion, being equipped with a pre-lifting section, a first reaction zone, a diameter-enlarged second reaction zone, and a diameter-reduced outlet zone which are coaxial with each other and arranged from the bottom up along the vertical direction in turn, wherein the a horizontal pipe is connected with the end of the outlet zone. Such reactor not only can control the technological conditions in the first and second reaction zones, but also can enable the feedstock oils having different properties to be stepwisely cracked so as to obtain the required products.
These patents construct the base patents of FCC Process for Maximizing Iso-Paraffins (MIP) and are widely applied. Currently they are applied in about 50 sets of FCC units, and achieve great economic and social benefits. Liquefied petroleum gas enriched in isobutane and gasoline enriched in iso-paraffins can be obtained according to the prior art. However, for the treatment of the feedstock oil of good quality for catalytic cracking, especially hydrogenated gas oil, the olefin content of the resultant gasoline and the isobutene content in the liquefied petroleum gas are lower, the product distribution is not adequately optimized, and the petroleum resources are not sufficiently utilized.